Orthogonal Frequency Division Multiplexing (OFDM) technology effectively improves spectrum utilization by utilizing orthogonal properties between subcarriers and allowing the subcarriers to overlap each other. Duration of data symbols on each subcarrier is greatly increased through a serial-parallel conversion of data flow, and Inter Symbol Interference (ISI) is effectively reduced by adding a cyclic prefix. Because each subcarrier has a narrow bandwidth, equalization operation can be performed on each subcarrier, which reduces complexity of receivers. OFDM technology has been widely used in Long Term Evolution (LTE) systems and WLAN systems.
In current development of wireless communication technology, as user number is rapidly increasing, the most important target of wireless communication technology is to increase system capacity and data transfer rate, so as to improve user experience. For these objectives, multi-antenna technologies have become a mainstream, among which Multiple-Input Multiple-Output (MIMO) technology is one of the major applications.
In MIMO, a plurality of transmitting antennas and a plurality of receiving antennas are respectively used as a transmitting end and a receiving end. Its basic idea is that, multi-antennas are respectively used at the transmitting end and the receiving end to improve spectrum utilization, communication quality and system capacity by using space-time processing technology to make full use of independent characteristics between the channels. The MIMO technology makes full use of the independent wireless channels between the transmitting end and the receiving end. At the receiving end, the plurality of different data flows transmitted from the transmitting antennas seems to have distinguishable spatial characteristics. Therefore, a combined MIMO channel between the transmitting end and the receiving end can be recognized as including N (N represents the smaller one of the antenna numbers at the transmitting and receiving ends) parallel sub-channels, and the capacity of the MIMO channel equals to a sum of capacities of the N sub-channels.
In a conventional MIMO-OFDM communication system, both the OFDM technology and the MIMO technology are used to improve spectrum utilization, reduce equalization complexity of receiver and improve the transmission rate of the system.
MIMO leads to a result that, a plurality of antennas simultaneously transmits a plurality of data flows on a single physical resource unit, and correspondingly, a plurality of antennas receive these data flows at the receiving end, so as to improve data transmission efficiency on the physical resource unit. In an OFDM system, each single physic resource unit is called a Resource Element (RE). The MIMO technology may multiplex N modulation symbols on a resource element, where N represents the number of data flows, which is greater than or equal to 2. Therefore, the spectrum efficiency can be multiplied by N times. N is determined by the number of antennas or ports at the transmitting end and the receiving end. The number of antennas or ports at both the transmitting end and the receiving end must be greater than or equal to N. The “Large delay CDD scheme” transmission mode, the “Closed-loop spatial multiplexing scheme” transmission mode and the “Dual layer scheme” transmission mode described in the standards of 3GPP TS 36.211 and 3GPP TS 36.123 are typical applications of MIMO technology in LTE system.
As we know, a high Signal to Noise Ratio (SNR) is needed in MIMO transmission. However, because there are interferences between the plurality of data flows, Block Error Ratio (BLER) decreases slowly as the SNR increases, which may affect data transmission rate.